Vehicle movement dynamics control systems continuously evaluate the vehicle movement dynamics state variables such as the velocity of the vehicle, the longitudinal acceleration of the vehicle, the lateral acceleration of the vehicle, the yaw angle, the yaw rate and, in particular, the steering angle. These vehicle movement dynamics state variables are sensed by sensors in the vehicle.
For reasons of cost, incremental sensors are frequently used for these purposes. Said sensors merely measure changes in the value of a variable but not the absolute value of said variable. The absolute value can be acquired by measurements of the changes in the variable relative to a permanently defined reference point. The point at which the variable assumes the value zero is usually used as such a reference point.
The incremental sensor itself cannot be used to detect the presence of the permanently defined reference point. For this reason, the sensor, or its evaluation logic, supplies a value which is referred to an undefined, randomly selected reference point and is used here as an uncentered value. This value is generally the absolute value of the variable to be measured which is present when the sensor is switched on but which is generally not known. The instantaneous uncentered value of the variable therefore actually differs from its instantaneous absolute value by the unknown absolute value which is present when the sensor is switched on.
In order to acquire the absolute value of the variable which is to be measured, the value of the defined reference point, expressed for example as a uncentered value of the variable to be measured, must firstly be acquired when a sensor which measures in a relative fashion is used. The instantaneous absolute value is then obtained from the difference between the instantaneous uncentered value and the uncentered value of the reference point.
The value of the reference point can be acquired, for example, by means of an additional sensor which outputs an index pulse if the reference point of the variable to be measured is present. The uncentered value of the variable to be measured which is sensed at the same time with the index pulse corresponds here to the value of the reference point. However, an additional sensor increases the costs for this sensor system of the vehicle and is therefore often not used. Instead, the absolute value which is present is acquired by means of an evaluation of the vehicle behavior which is based on vehicle models and which is described by the measured values of further sensors which measure in an absolute fashion. The value of the reference point can then be inferred from the absolute value which is acquired.
In known methods for determining the value of the reference point of a variable which is measured with an incremental sensor, a defined driving situation in which the vehicle behavior is evaluated by means of the further measured values is generally predefined. For example, incrementally measuring steering angle sensors are particularly frequently used. In this context, for example the uncentered value of the zero point of the steering angle which corresponds to the straight-ahead position of the steerable vehicle wheels can be acquired while straight-ahead travel is detected using other measured values, for example the measured yaw rate of the vehicle and the measured lateral acceleration. The value of the zero point actually corresponds here to the uncentered steering angle value which is present during straight-ahead travel.
In known methods, the value of the reference point cannot be detected until the defined driving situation occurs. However, it often takes a relatively long time until the defined driving situation occurs for the first time. However, up to this time, it is not possible to acquire an absolute value of the variable which is to be measured and the vehicle systems which use this absolute value as an input variable are not operational, or are only operational to a limited degree.